Field bus systems are frequently used in the industry for enabling communication between different units in control systems. Various types of field buses have been developed by different companies and organizations all over the world. The term field bus covers many different industrial network protocols. Generally, industrial field buses may be sub-divided into different categories depending on the capabilities they offer. The simplest field bus protocols are designed primarily for an on/off interface used for e.g. valves, proximity sensors, and limit switches, while more complex field bus protocols offer handling of large amounts of data as when a computer communicates with a PLC (Programmable Logic Controller). Examples of filed buses of different complexity used today are CONTROLNET, PROFIBUS, DEVICENET, CANBUS, and LONWORKS.
When connecting e.g. a PLC to a specific field bus, the manufacturer of the PLC must provide an interface to the filed bus in order to enable reception and transmission of data via the field bus network. Since the number of available field bus protocols is extensive, and most of the protocols are not compatible, neither from a physical nor a communicative point of view, it is expensive and cumbersome to provide e.g. PLCs for each and every possible filed bus system.
One solution to the problem above is to provide a communications module, which acts as an interfaces between the PLC and the field bus network. The interface between the communications module and the PLC may hence be fixed and standardized while the interface towards the filed bus network may vary from module to module.
However, devices, such as the PLC, which are to be connected to the field bus network via the communications module are, depending on their design, best suited for communication in either a serial or parallel mode. For example, in a device that transfers large amounts of data a parallel interface may be the most efficient due to the reduced load on the communications circuitry as regards processing speed in the device. Other special design parameters, such as cost or size of the device may require a device to communicate in either a serial or parallel mode.
For optimizing the performance of a communications module which is to act as an interface between the device and a field bus system, the communications module should be able to communicate in both serial and parallel mode towards the device. However, problems arise as to the set-up of communications module when it is connected to the device, i.e. should the communications module communicate via a parallel or serial interface, and if serial mode is best suited for communication, what baud rate should be used for the data transfer?
Consequently, there is a need for an interface between the device and the communications module that enables the communications module to fast and unfailingly determine which communication mode to operate in.